Unusually large masses of green filamentous algae were recently discovered in the Volga River, European Russia. Macroscopically, their thick, slightly curled, bright-green filaments are distinct from the commonly occurring Cladophora glomerata. Microscopic examination showed that the filaments were entirely unbranched and composed of relatively large cells containing up to 170 nuclei, features characteristic of Chaetomorpha. However, molecular phylogenetic analyses based on nuclear rDNA sequences placed these algae within the genus Rhizoclonium, leading to its description as a new species, Rhizoclonium volgense sp. nov. Unlike many other Rhizoclonium species, R. volgense lacks lateral rhizoids, is completely unbranched, and becomes unattached in later growth stages. It was consistently found to form free-floating mats, although the presence of primary rhizoids in sporelings indicates an initially attached phase. Molecular analyses indicate a close relationship with taxa from saline environments. Currently, the new species appears to be restricted to the regulated reservoirs of the upper Volga River, where fluctuating hydrological conditions may influence its distribution. Its absence from earlier studies suggests either historical oversight or a recent introduction.